Controlled assembly of graphene shells encapsulated gold nanoparticles and their integration with carbon nanotubes

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Controlled assembly of graphene shells encapsulated gold nanoparticles and their integration with carbon nanotubes Nitin Chopra *, Junchi Wu, Larry Summerville Metallurgical and Materials Engineering, Center for Materials for Information Technology (MINT), Box 870202, The University of Alabama, Tuscaloosa, AL 35401, USA

A R T I C L E I N F O

A B S T R A C T

Article history:

Controlled growth and uniform patterning of graphene/carbon shells encapsulated gold

Received 20 February 2013

nanoparticles (GNPs) on silicon wafer or on high curvature carbon nanotubes (CNTs) is

Accepted 29 May 2013

reported here. This was achieved by utilizing patterned gold nanoparticles with controlled

Available online 7 June 2013

sizes (30–600 nm) via gold film dewetting process. Surface-oxidized and patterned nanoparticles were used as sacrificial catalysts for the chemical vapor deposition (CVD) growth of graphene/carbon shells. The shell morphological evolution and thickness as well as surface migration of nanoparticles during the CVD process were studied as a function of the gold nanoparticles size. Reduced surface migration and coalescence was observed for gold nanoparticles after the CVD growth and this was attributed to the initial formation of graphene/carbon shells as well as stable dispersion of the dewetted gold nanoparticles. It is proposed that graphene/carbon shell growth was controlled by Ostwald’s ripening, surface gold oxide, and reducing CVD growth environment. Furthermore, complex heterostructures based on CNTs coated with GNPs were fabricated by dewetting Au films on CNTs and followed by surface oxidation and CVD growth steps. CNTs successfully survived multiple processing steps and selective growth of graphene shells around Au nanoparticles was achieved and studied using microscopic and spectroscopic methods.  2013 Elsevier Ltd. All rights reserved.

1.

Introduction

Metallic nanostructures, due to their unique size-dependent properties and light-matter interactions, are important for photonics and sensing applications [1]. For example, gold (Au) nanoparticles exhibit distinct surface plasmon peaks depending on their shape and size as well as solution environment [2,3]. However, single component Au nanoparticles also demonstrate problems such as aggregation, dissolution in solvents, and limited surface chemistry [4]. To overcome these, heterostructured configurations of Au nanoparticles are of interest. For example, core/shell nanoparticles with a

dielectric core (e.g., silica) encapsulated in Au shell, nanoimprinted Au-coated polymer fingers, and graphene-Au composites are interesting heterostructured configurations with tunable physical, chemical, and optical properties [5–9]. Among these graphene-based heterostructures demonstrate potentially promising plasmonics and chemical effects [8,9]. However, their fabrication involved tedious synthesis approaches requiring multiple processing steps, such as transfer of graphene from one substrate to another and subsequent Au film/nano-island coating, or utilizing e-beam lithography to pattern plasmonic nanostructures onto graphene films [8,9]. Apart from being serial approaches, such

* Corresponding author: Fax: +1 205 348 2164. E-mail address: [email protected] (N. Chopra). 0008-6223/$ - see front matter  2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2013.05.055

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multiple processing steps can damage the graphene surface, quality, and affect its properties. Curved or rippled graphene can improve plasmonic behavior of Au nanoparticles and can impart greater chemical reactivity to the hybrid nanosystem [10–12]. Of particular importance is surface passivation and encapsulation of Au nanoparticles in a robust and sub-10 nm thick graphitic/carbon cage or shell [13–16], where the shell is not detrimental to the properties (e.g., electronic and optical) of the encapsulated nanoparticle. Furthermore, it provides unique and diverse avenues for surface chemistry and hierarchical assembly. Recent reports by the authors demonstrated a facile and scalable xylene chemical vapor deposition (CVD) approach to grow graphene shells encapsulating Au nanoparticles (GNPs), where the sub-2 nm thin and non-turbostratic shells exhibited inter-planer spacing consistent with the c-axis spacing between the graphene layers [17–19]. In addition, fluorescence sensing and bioanalysis using carboxylic group terminated on the surface of GNPs was demonstrated [19]. In regard to GNP growth, the surface plasma oxidized Au nanoparticles served as a sacrificial mask and catalyst for the CVD growth of graphene shells resulting in controlled shell thickness (1–20 nm) [19]. The graphene shell growth mechanism involved electron transfer process at high CVD growth temperatures, where unstable surface gold oxide converted to Au(0) by accepting electrons from the incoming carbon feed, which in turn resulted in sp2–sp3 hybridized graphene shells around Au nanoparticles [17,19,20]. It was also proven that the non-oxidized Au nanoparticles did not result in graphene shells [19,21]. A total yield of 2.63 · 104 cm2 of graphene shell surface area per unit gram of Au nanoparticles with 35.8% coverage of the substrate was attained [17]. However, this study [17] highlighted limitations of the GNP growth approach including, significant aggregation and surface migration of surface-oxidized Au nanoparticles during the CVD growth process leading to large variations in the diameter of GNPs. This must be overcome in order to realize nanodevices from these novel hybrid nanoparticles. The present study is based on the hypothesis that a minimal surface energy configuration of Au nanoparticles would minimize surface migration, coalescence, and aggregation of the nanoparticles during CVD growth of graphene shells. In this regard, dewetting of Au films is a promising approach [22] that would facilitate formation and uniform dispersion of GNPs on complex substrate geometries such as high curvature carbon nanotubes (CNTs). Here, we report high temperature dewetting of Au films resulting in controlled morphology and dispersion of the Au nanoparticles patterned onto silicon (Si) substrate/wafer. These Au nanoparticles were further surface plasma oxidized and utilized for the graphene shell growth in a xylene CVD process. The graphene shell thickness, crystallinity, and surface migration of Au nanoparticles during the growth process were studied as a function of the size of the surface-oxidized Au nanoparticles. Furthermore, suitable Au film dewetting conditions were selected for fabricating heterostructures comprised of CNTs coated with Au nanoparticles (CNT–Au nanoparticles heterostructures). The selective growth of graphene shells around Au nanoparticles coated onto CNTs was accomplished and studied.

2.

Experimental

2.1.

Materials and methods

Sulfuric acid (H2SO4, 95–98% mol) was bought from VWR (West Chester, PA). Hydrogen peroxide (H2O2, 35% w/w) was purchased from Alfa Aesar (Ward Hill, MA). Si wafers (100, n-type) were purchased from IWS (Colfax, CA). Ferrocene (Fe(C5H5)2, 98%) was bought from Sigma–Aldrich (St. Louis, MO). DI water (18.0 MX-cm) was obtained using a Barnstead International DI water system (E-pure D4641). All chemicals were used without further purification. Sputtering target (Au) and ATC ORION sputtering system were purchased from AJA International, Inc. (North Scituate, MA). The sputtered Au film was annealed in CMF-1100 (MTI Inc, Redmond, CA) compact muffle furnace. Oxygen plasma treatment was performed in a Nordson March Jupiter III Reactive Ion Etcher (Concord, CA). Graphene growth was conducted inside a Lindberg blue three-zone tube furnace (Watertown, WI). Quartz tube was purchased from ChemGlass (Vineland, NJ). Xylene (o-, m-, p-isomers) and syringe injector were obtained from Fisher Scientific (Suwanee, GA). Gas flow rate of all annealing and CVD processes were controlled by Teledyne Hasting powerpod 400 mass flow controllers (Hampton, VA). Thermocouples and temperature controllers were bought from Omega Engineering (Stamford, CT). H2 (UHP grade, 40% balanced with Ar), O2 (UHP grade, 5% balanced with Ar), N2 (UHP grade), and Ar (UHP grade) gas cylinders were purchased from Airgas South (Tuscaloosa, AL).

2.2.

Deposition and dewetting of gold (Au) films

Si wafer was soaked in a mixture of H2SO4 and H2O2 (v/v 5:1) at 100 C for 30 min. Subsequently, the wafer was rinsed with large amounts of DI water and dried in air. The piranhacleaned Si wafer was sputter-coated with Au thin film at 2 kV, 40 W, and 0.045 Torr for 2 min. This Au-coated substrate was annealed at 700 C for 2 h to result in Au nanoparticles patterned on the substrate via a dewetting process. To study the influence of different parameters on surface migration and size and shape distribution of Au nanoparticles after dewetting, systematic studies were performed by varying Au deposition time (Au film thickness) as well as annealing temperatures and times (Table S1, see Supplementary information, A-series samples). For these experiments, only one variable was changed while keeping all the other experimental conditions fixed.

2.3. Synthesis of nanoparticles (GNPs)

graphene

shells

encapsulated

Au

Samples #1A–6A (Table S1, see Supplementary information) were utilized for this study to understand the influence of Au nanoparticle size on the growth of graphene shells. These samples were listed as B-series sample (Sample #1B–6B, Table S4, see Supplementary information). Prior to the growth of graphene shells, substrates with patterned and dewetted Au nanoparticles were surface plasma oxidized for 30 min (160 W and 600 mTorr). The substrates were placed in the center of the quartz tube equipped with precursor and gas lines

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for Ar/H2 flow. Xylene was utilized as the carbon source. It was injected through a syringe injector into a preheated zone (220 C) at the rate of 45 mL/h for 2 min and subsequently transported into the reaction zone (675 C) inside the quartz tube furnace. The xylene flow rate was then reduced to 1 mL/ h after H2 mixed with Ar (Ar/H2 = 0.85 LLM/0.3 SLM or 10% v/v H2) was introduced in the CVD reactor. The CVD reaction was continued for 1 h after which H2 and xylene were discontinued, and the furnace was cooled down under Ar flow. In order to understand step-by-step chemical and surface characteristics of the Au nanoparticles and GNPs, two sets of samples (Cseries samples) were prepared: (1) Only blank Si wafers (as control samples) and (2) Si wafers with sputtered Au films (sputtering duration: 2 min). Each set of samples was analyzed by X-ray photoelectron spectroscopy (XPS) after annealing (700 C, 2 h), plasma oxidation (160 W, 600 mTorr, 30 min), and CVD growth of graphene shells (675 C, 1 h). The sample numbering is described in Table S5.

were all measured using Adobe Photoshop software by exceeding number of counts by 200 for each sample studied here. X-ray reflectivity (XRR) data of samples were obtained with a Philips X’Pert Diffractometer (X’pert Powder, Cu Ka radiation, 35 mA and 40 kV). XRR measurements were used to study the thickness of deposited Au films. Raman spectra were collected using Bruker Senterra system (Bruker Optics Inc. Woodlands, TX) equipped with 785 nm laser source at 10 mW laser powers and 100· objective. The integral time and co-additions were set as 15 s and 2, respectively. X-ray photoelectron spectroscopy (XPS) was utilized to analyze elements and their chemical states on samples’ surface. XPS data was gathered by a Kratos Axis 165 using a mono-aluminum gun. The analysis spot was set as ‘‘slot’’ with >20 lm aperture and 19.05 mm iris setting.

2.4. Synthesis of carbon nanotubes (CNTs) and CNT–Au nanoparticles heterostructures

Fig. 1 outlines the approach for controlled patterning and dewetting of Au nanoparticles on a Si wafer or CNTs, their surface oxidation, and utilization for the growth of GNPs. This study is divided into three parts: (1) Systematic studies for understanding the morphological evolution of Au nanoparticles by dewetting of Au films coated on substrates such as Si wafer or CNTs, (2) growth of GNPs as a function of the size of the core Au nanoparticles dispersed on Si wafer, and (3) growth of GNPs directly on CNTs.

Multiwalled carbon nanotubes (MWCNTs or CNTs) were synthesized in a floating catalyst CVD process using ferrocene and xylene as catalyst and carbon source, respectively [23– 25]. A liquid mixture of ferrocene and xylene (Fe:C = 0.75% mol) was injected through a syringe injector into a pre-heated zone (220 C) and subsequently transported into the reaction zone (675 C) in the center of a quartz tube furnace. Meanwhile, the gas flow of H2 (oxygen scavenger) in carrier gas Ar (10% v/v) was also introduced into the CVD furnace. The CVD growth reaction continued for 2 h before the furnace was cooled down in Ar gas flow. The as-synthesized CNTs were collected as black powder from the inner walls of the quartz tube, stored in a solvent, and were dispersed on piranha-cleaned Si wafer by drop-casting method. After drying of the CNT substrate, the Au film was sputtered (2 kV, 40 W, and 0.045 Torr) on this substrate for 30 s duration. Finally, Au nanoparticles were nucleated via dewetting process on CNTs by annealing this substrate at 400 C for 2 h in N2 atmosphere. In a similar xylene CVD approach as described above, graphene shells were also grown on CNT–Au nanoparticles heterostructures dispersed on a Si wafer. In this case, the heterostructures were surface plasma oxidized for 10 min prior to the CVD growth to result in graphitic shells around Au nanoparticles. This plasma oxidation duration prevented CNT damage and decomposition.

2.5.

Characterization methodologies

The morphology and Energy-dispersive X-ray spectra were obtained by Field Emission Scanning Electron Microscope (FE-SEM, JEOL-7000, equipped with Oxford EDX detector). High-resolution transmission electron microscope (HR-TEM, Tecnai FEI-20) was used to obtain TEM images for all the nanomaterials at various stages of growth. X-ray diffraction (XRD) data of samples were recorded with a Philips diffractometer (XRG 3100, Cu Ka radiation, 35 mA and 40 kV). The diameters of nanoparticle and nanotubes, graphene shell thickness, inter-particle and graphene inter-layer spacing

3.

Results and discussion

3.1. Morphological evolution of gold films in a dewetting process Controlling aggregation of Au nanoparticles on the large-area substrates by way of self-assembly chemistry approach remains a daunting challenge [4,26]. Thus, high temperature annealing or dewetting that involves minimization of surface and strain energy of the metal films is a promising approach [22,27,28]. The basic mechanism for the microstructure evolution during dewetting involves the void/defect formation and vacancy nucleation at the film/substrate interface [29,30]. As the annealing process continues, the growth of voids took place along the area of high stress such as grain boundaries and resulted in minimal surface energy shape and size of the metal islands or nanoparticles [31,32]. Table S1 (Supplementary information) shows the systematic study performed to control the dewetting of Au films to result in Au nanoparticles with different shapes and sizes. SEM and EDS analysis of a thin Au film that was sputtered onto a piranha-cleaned Si wafer for 2 min is shown in Fig. S1 (Supplementary information). The as-deposited Au films showed polycrystalline grains and exhibited (1 1 1) fcc Au peaks while after dewettitng, (2 2 2) fcc Au peaks also emerged (Fig. S2A, see Supplementary information). By controlling the Au sputter deposition conditions, it was possible to control the thickness of as-deposited Au films. XRR (Fig. S2B, see Supplementary information) showed a linear increase of film thickness (h) with increasing deposition time (from 8 nm for 30 s to 45 nm for 180 s).Table S2 (Supplementary information) summarizes the results of the dewetting study corresponding to different conditions. A definitive increasing trend in the Au

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CVD growth of graphene shells

Dewetting

Graphene growth

C

Au nanoparticle formed by dewetting of Au film

B

CVD growth of graphene shells

Plasma oxidation

Graphene shells encapsulated Au nanoparticle

Oxidized AuNP

Dispersion of CNTs

Graphene growth

Au film deposition

Dewetting CVD growth of graphene shells

Au nanoparticles decorated CNT

Fig. 1 – Schematic illustration of dewetting of the Au films on (A) Si wafer or on (B) CNTs to result in Au nanoparticles. (C) Surface oxidation of dewetted Au nanoparticles for the CVD growth of GNPs.

nanoparticle size (from 30 to 662 nm) and inter-particle spacing (from 25 nm to 1952 nm) was observed for samples #1A-6A (Fig. S3, Table S2, see Supplementary information). These samples corresponded to increasing Au sputter deposition time (or h). Based on h and Au nanoparticle size (D) and inter-particle spacing (S) for samples #1A–6A, the following relationships were obtained: D = 1.228h1.63 and S = 0.184h2.39 (Fig. S4, see Supplementary information). Such a dependence for D and S with h has been known to be due to large capillary flow and velocities with short patterning lengths leading to robust and stable nanoparticles after the dewetting process [33]. According to thin film hydrodynamic theory, this was attributed to large surface tension and low viscosity at the dewetting temperatures [33]. In regard to annealing temperatures (from 150 to 1100 C), dewetting process was initiated at or above 150 C, nanoparticles or island formation began above 600 C, and uniform dispersion of nanoparticles was observed around 1100 C (Fig. S5, see Supplementary information). The decrease in Au nanoparticle size could be attributed to the evaporation of Au droplets at elevated temperatures [34]. With increasing dewetting time (t = 10 min–10 h), the average size of nanoparticles (D) reduced (from 348 nm to 254 nm) and the complete dewetting of Au films into Au nanoparticles occurred within 10 min of annealing at 700 C (Fig. S6, see Supplementary information). Apart from the possibility of Au droplet evaporation [34], this decrease in average size of Au nanoparticles as a function of dewetting time followed von Smouluchowski kinetic rate equation (D = kt a), where k and a are constants [35]. The low value of a (0.0851, Fig. S7, see Supplementary information) for the dewetted Au nanoparticles indicates a strong nanoparticle–surface interfacial adhesion as compared to other materials systems [35–37]. Such low value of a or

strong interfacial adhesion is suggestive that Au atom migration from clusters/nanoparticles could have been occurring during the dewetting process [37] and led to reduction in Au nanoparticle size. This was also the reason for observing large standard deviations in the average nanoparticle sizes for these samples (Table S3, see Supplementary information). Overall, these systematic studies indicated that suitable dewetting conditions to control the size and dispersion/patterning of Au nanoparticles corresponded to sample #1A–6A (Table S1, see Supplementary information).

3.2. Synthesis of nanoparticles (GNPs)

graphene

shells

encapsulated

Au

The above described optimized dewetting conditions (sample #1A–6A, Table S1, see Supplementary information) led to stable nanoparticle configurations that should survive multiple processing steps including high temperature CVD growth of graphene shells. This opens an opportunity for understanding the relationship between the Au nanoparticle size and post-CVD graphene shell characteristics (thickness, amorphous content, strains, and inter-planar spacing). Prior to graphene shells growth, the substrates with dewetted Au nanoparticles were surface plasma oxidized [17,19]. The plasma oxidation step is a dry oxidation approach that allowed for controlled surface oxidation of Au nanoparticles without affecting their dispersion on the substrate [17,19]. This surface gold oxide acted as a sacrificial catalyst for the growth of graphene shells and without this surface oxidation step, no graphene/carbon shell growth occurred around Au nanoparticles [19]. Thus, the substrates with surface-oxidized Au nanoparticles were further subjected to CVD growth of graphene shells resulting in GNPs on the substrate. Table S4

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(Supplementary information) summarizes details of various samples such as nanoparticle sizes and inter-particle spacing before and after the CVD growth, graphene/carbon shell thickness, and interplaner spacing of carbon layers in GNPs. In order to prove that surface gold oxide was a sacrificial catalyst for the growth of graphene shells, detailed XPS analysis was performed. Fig. S8 shows XPS survey scans for the samples after various treatments such as annealing, plasma oxidation, and CVD growth process. These treatments were performed on blank Si wafers (control samples) and Si wafer sputtered with Au (sample #1C–8C, Table S5, see Supplementary information). Elements such as Si, O, C, and Au were analyzed as indicated in Tables S6–S9 and Figs. S8–S15 (Supplementary information). Piranha-cleaned Si wafer (sample #1C–4C) before and after treatments exhibited Si and SiO2 peaks (Table S6, Fig. S9) [38] indicating the presence of SiO2 layer. In regard to Si wafer with sputtered Au film (sample # 5C) and after various treatments (samples #6C–8C), SiO2 signal was consistently observed (Fig. S10). Similarly, oxygen signals were detected for all the samples irrespective of the sample type and treatment (Table S7, Figs. S11 and S12). Oxygen signals could be attributed to the base Si substrate with native oxide layer. Thus, XPS analysis using oxygen signals would not allow understanding the role of surface gold oxide in the growth of graphene shells. The most suitable approach will be to analyze peaks for Au and gold oxide before and after CVD growth of GNPs. Deconvolution of Au 4f peaks clearly indicated that Si wafer sputtered with Au film (sample #5C) and dwetted/annealed Au films (sample #6C) only exhibited metallic Au peaks at 84.5 eV and 88.5 eV (Table S8, Fig. S13A and B) [39]. After plasma oxidation, two new peaks corresponding to gold oxide at 87.0 eV and 90.0 eV emerged (Fig. S13C), which indicated the formation of gold oxide on the surface of Au nanoparticles [40]. Finally, after CVD growth of GNPs, these gold oxide peaks disappeared (Fig. S13D). This confirmed that gold oxide was a sacrificial catalyst during the CVD growth and facilitated the formation of graphene shells as proposed earlier [19]. Carbon contamination is commonly present in XPS, thus a strong C peak was observed for all the samples (Table S9, Figs. S14 and S15) but deconvoluted C peaks corresponding to GNPs showed p–p* transition loss peak (293.87 eV, Fig. S15D). This confirmed the presence of graphitic carbon shells around Au nanoparticles [25]. The deconvoluted peak located at 284.8 eV corresponds to C 1s peak and peaks located at 286 and 288 eV could be attributed to carbon–oxygen links [41,42]. Fig. 2A–F shows high resolution TEM images of GNPs indicating uniform shell formation with corresponding carbon lattice spacing. The thickness of the graphene or carbon shells increased (2–11 nm, Sample #1B–6B, Table S4, see Supplementary information) with increasing core Au nanoparticle size (30–660 nm). Thicker graphene or carbon shells also embedded small-sized Au nanoparticles (3–4 nm, Figs. 2C–F and Fig. S16, see Supplementary information) with lattice spacing of 0.22 nm, corresponding to (1 1 1) fcc Au planes (Table S10, see Supplementary information). As the core Au nanoparticle size increased beyond 200 nm (Sample #3B–6B, Table S4, see Supplementary information), amorphous carbon formation was also observed within the shells (Fig. 2C–F). The morphological evolution of shells around Au

nanoparticles is schematically shown in Fig. 2G. Using microscopic characterization results, average sizes and inter-particle spacing of Au nanoparticles before and after CVD growth as well as average thickness and c-axis inter-planer spacing for the graphene or carbon shells were estimated (Fig. 3, Table S4, see Supplementary information). The average sizes of the core Au nanoparticles increased and larger-sized Au nanoparticles resulted in greater increase in their sizes after the CVD growth (Table S4, see Supplementary information). However, the size difference for dewetted Au nanoparticles before and after CVD growth is lower than chemically patterned Au nanoparticles that coalesced significantly in our previous report [17]. The smaller the size of dewetted Au nanoparticles, lesser is their tendency to coalesce after the CVD growth (Fig. 3A, Sample #1A). Another interesting aspect is inter-particle spacing for GNPs that was negligibly changed with respect to dewetted Au nanoparticles before the CVD growth (Figs. 3B and S17, see Supplementary information). This suggests sinter-resistance characteristics of GNPs. Thus, as soon as graphene or carbon shells grew around Au nanoparticles, it limited the surface migration ability of the encapsulated Au nanoparticles. Despite limited surface migration leading to increasing Au nanoparticle sizes after the graphene or carbon shells growth, the uniform dispersion of these nanoparticles on the substrate was well-maintained. This also suggests that surface migration of small-sized Au nanoparticles embedded within graphene or carbon shells must have occurred within a short duration at the beginning of the CVD growth. Overall, the dewetting followed by CVD growth described here overcomes the challenges of electron irradiation-based approaches for GNP growth [13,14,35] by resulting in uniform dispersion, size-controlled, and negligible aggregation of produced GNPs patterned on the Si substrate. This is advantageous for applications of GNPs, where the large-area and sinter-resistant (high temperature stable) nanodevices can be laid down. In order to quantitatively explain the surface migration kinetics, the observations for nanoparticles before and after GNP growth were used to estimate the mass transfer surface diffusion coefficient (Ds) for 30 nm diameter nanoparticles (sample #1, Table S4, Fig. S18A, see Supplementary information). This diffusion coefficient was found to be 2.1 · 10 17 m2/s [22,43–45] and corresponded to a high activation energy for surface migration (Q) 370.45 kJ/mol [46] (Fig. S18B, see Supplementary information). The calculations for Ds and Q for surface migration are detailed in the supplementary information. The estimated Ds was much lower than the reported room temperature values (10 12 m2/s) for Au, Sb, and Cu on HOPG substrate [37,47]. It has been reported earlier that mass transfer-based surface diffusivity (or Ds) for metallic clusters has a strong dependence on the cluster size as well as substrate-cluster interaction (or contact angle) [37]. Furthermore, it was proposed that Q for surface migration of the metallic clusters depends on the size of the clusters [37] and related to Ds. Thus, assuming that Au nanoparticles are spherical in shape, Ds and Q were plotted as a function of the contact angle (h) and size of Au nanoparticles during CVD growth process (Fig. S18, Equations S1–S3, see Supplementary information). These basic calculations suggest that largersized nanoparticles have lower Q and higher Ds at the CVD

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C ~0.41 nm ~0.43±0.1 nm

~0.37 nm

2 nm

10 nm

D

5 nm

E

F ~0.42 nm

~0.45±0.1 nm ~0.40±0.1 nm 5 nm

G

5 nm

10 nm

Dewetted Au nanoparticles or islands

#1A

#2A

#3A

#4A

#5A

#6A

Graphene shells encapsulated Au nanoparticles or islands

#1B

#2B

#3B

#4B

#5B

#6B

Fig. 2 – TEM images of GNPs corresponding to (A) Sample #1B, (B) Sample #2B, (C) Sample #3B, (D) Sample #4B, (E) Sample #5B, (F) sample #6B. (G) Schematic illustrating the morphological evolution of GNPs as a function of nanoparticle size. Samples #1B–6B (Table S4 see Supplementary information) were prepared using samples #1A–6A (Table S1, see Supplementary information).

growth temperature. This further support our observations of increasing Au nanoparticle size after CVD growth with increasing initial size of the surface-oxidized Au nanoparticles (Table S4, see Supplementary information). Similar dependence of Q on the Cu and Au clusters sizes has been estimated indicating that larger clusters migrate on surface easier than smaller ones [37,48]. Fig. 3C shows that with increasing size of dewetted Au nanoparticles, the graphene or carbon shell thickness increased from 2 nm (for sample #1B) to 11 nm (for sample #6B). The c-axis inter-planar spacing was larger than 0.34 nm (0.37 nm for sample #1B) and showed an increasing trend with increasing Au nanoparticle size, reaching as high as 0.4–0.45 nm corresponding to sample #4B–6B (Fig. 3C, Table S4, see Supplementary information). In this regard, radius of curvature (r = r) of GNPs is a critical parameter to understand while keeping c-axis spacing (0.34 nm) of perfectly flat graphene sheets (r = infinity) as a reference [49]. Thus, as the radius of GNPs increased, the strains fields within the graphene layers are expected to be closely matching

with or tending towards that of flat graphene sheets. Combining this argument with the fact that larger GNPs also incorporated amorphous carbon in the shells, such GNPs should have much relaxed graphene stacking as indicated by larger c-axis inter-planar spacing (>0.4 nm) for sample #3B–6B. On the other hand, GNPs with smaller radius of curvature (sample #1B) would exhibit highly strained (or compressively stressed) graphene layer stacking within the shells, where c-axis interplanar spacing was smaller (0.37 nm) and near to that of flat graphene sheets. This can be further explained with the help of Raman spectra (Fig. 3D) for the GNPs where notably the perfectly flat graphene sheets exhibited a G-band Raman shift around 1580 cm 1 [50]. Fig. 3E and Table S4 (Supplementary information) shows a blue shift (i.e., towards larger wavenumber) of G band peak location for GNPs with respect to flat graphene sheet. As mentioned, the strain fields within a graphene shell lattice would increase for lower radius of curvature [51], which can now explain the reason for the largest blue shift (w.r.t flat graphene sheet) of G band peak location for samples #1B and #2B corresponding to 1598 and

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Au nanoparticles interparticle spacing (nm)

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A

800 600 400 200 0 0

1500 1000 500 0

30 60 90 120 150 180 (#1) (#2) (#3) (#4) (#5) (#6)

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100 80 60

30 60 90 120 150 180 (#1) (#2) (#3) (#4) (#5) (#6) Au film deposition time (s)

D-band -1 1314.2 cm G-band -1 Sample 1598.0 cm #1B

D

40 20 0 1000 1200 1400 1600 1800 -1 Raman shift (cm )

ID/IG

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-1

1605 1600 1595 1590 1585 1580 1575 1570

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G-band location (cm )

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Thickness of graphene shells (nm)

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Graphene shells interplanar spacing (nm)

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C

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Au film deposition time (s) 18 15 12 9 6 3 0

Before GNP growth After GNP growth

2500

30 60 90 120 150 180 (#1B) (#2B) (#3B) (#4B) (#5B) (#6B) Au film deposition time (s)

Fig. 3 – Histograms showing (A) size and (B) inter-particle spacing distributions for Au nanoparticles before (gray) and after (red) CVD growth of GNPs. (C) Average thickness (black) and average c-axis inter-planar spacing (blue) for graphene shells in GNPs. (D) A representative Raman spectrum for GNPs (sample #1B, Table S4, see Supplementary information). (E) Average ID/IG ratio and G-band peak location estimated using Raman spectroscopy. Note: x-axis shows the Au film deposition time (in seconds) prior to dewetting process as well as corresponding GNP samples (Sample #1B–6B, Table S4, see Supplementary information). Dotted line in (C) and (E) represent the standard c-axis inter-planer spacing (0.34 nm) and G-band peak position (1580 cm 1) corresponding to flat graphene sheet, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 1602 cm 1 (Fig. 3E), respectively. This argument is strengthened by consistent red shift (w.r.t. sample #1B) of G-band peak position, with increasing radius of curvature of GNPs, towards lower wavenumbers (or near G-band peak for perfectly flat graphene sheets), and reached as low as 1590 cm 1 (sample #6B). ID/IG ratio essentially remained the same for all the samples indicating proportional increase in the disordered and graphitic content of the shells, irrespective of the shell thickness (Fig. 3E). Based on the above analyses and observations, there are several competing processes that must be synchronized together for developing an understanding of the GNP growth mechanism. The authors propose that surface gold oxide content on the core Au nanoparticles, Ostwald’s ripening, surface migration of Au species, and CVD reactions play a critical role in the evolution of different morphologies of graphene or carbon shells as a function of core Au nanoparticle sizes. Thus,

the growth mechanism of graphene or carbon shells on the surface-oxidized Au nanoparticles can be explained in three parts as described next. Firstly, knowing that the oxidation kinetics is slower for larger-sized Au nanoparticles [52], the thickness or content of surface gold oxide will be lower for larger-sized Au nanoparticles as compared to smaller-sized ones for the fixed plasma oxidation duration (30 min). This would lead to thinner graphitic shells for larger-sized Au nanoparticles. However, since the CVD growth duration remained fixed (1 h) for all the samples, larger-sized Au nanoparticles with thinner surface gold oxide exhibited amorphous carbon (due to non-catalytic carbon decomposition, Fig. 2) as compared to the smaller-sized nanoparticles. Secondly, in the initial part of CVD growth of graphene shells, Ostwald’s ripening of the Au nanoparticles, surface migration of Au species on the substrate, and solubility of carbon in Au were three critical aspects. Carbon solubility is

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decreased with increasing Au nanoparticle size [53,54] and partially explains the amorphous carbon formation within the shells as the Au nanoparticle size is increased (Fig. 2). This combined with weak Au-C bonding further led to unstable alloying of carbon in Au cluster and hinders the formation of the nucleation cap [53]. However, in case of GNPs, the presence of surface gold oxide favors the formation of this cap but since the surface gold oxide content for larger nanoparticles is lower than smaller nanoparticles [52], the oxide is rapidly utilized for the larger ones. Thus, with the consumption of surface gold oxide within CVD duration of 1 h and anchoring of much smaller Au nanoparticles (<5 nm) on already formed graphene shells, the formation of graphitic carbon is inhibited and amorphous carbon formation is favored. Furthermore, it is known that pure gold poorly assists in the dissociation of incoming hydrocarbon feed and can result in amorphous carbon formation in the temperature range of 600–700 C [53,55]. On the other hand, due to Ostwald’s ripening effect [35,56,57], the larger nanoparticles became larger at the expense of the smaller nanoparticles. This facilitated surface migration of Au species but at the same time, graphene shell growth took place in the initial CVD duration. The latter restricted the migration of Au species and if they migrated and were in transit, they anchored to the graphene or carbon shell already encapsulating another Au nanoparticle. As the growth continued, more carbon decomposition on such nanoparticles took place and embedded them within the shells. Supported by the reasoning above, the formation of amorphous carbon on these small-sized embedded Au nanoparticles, assuming them to be non-oxidized at CVD growth temperatures, cannot be ruled out [19]. Also observed for larger Au nanoparticles (Fig. S16, see Supplementary information, shown by arrows), the broken graphitic shells on the outer edge of GNPs must be due to the low density amorphous carbon leading to volume expansion within the shells and

pushing or lifting the initially grown graphitic carbon regions as well as causing their rupture. Lastly, CVD reactions must be considered in conjunction with surface oxide content of the Au nanoparticles. Two competing reactions must have occurred; reaction of H2 with the surface gold oxide and the electron transfer reaction between incoming carbon feed and surface gold oxide [17]. For the given CVD duration, with sufficient surface gold oxide (smaller nanoparticles), the forward reaction with reducing H2 environment hinders the electron transfer process and results in thin graphene shells (<5 nm) [17]. On the other hand, the surface gold oxide content is lower for larger-sized Au nanoparticles [52] and the reaction with hydrogen will compete with electron transfer based graphene shell growth. With a fixed CVD growth duration, complete consumption of surface oxide on larger nanoparticles takes place faster than a smaller and leads to non-catalytic amorphous carbon formation as explained above. The growth mechanism of the graphene or carbon shells is schematically represented in Fig. 4.

3.3. Synthesis of carbon nanotubes (CNT)-Au nanoparticles heterostructures and graphene shells encapsulated Au nanoparticles (GNPs) decorating CNTs Decorating nanoparticles onto the surface of high curvature substrates such as nanowires and CNTs is of interest due to their combined properties and ability to minimize nanoparticle aggregation [25,58,59]. However, certain major challenges remain such as uniform decoration of nanoparticles on CNTs without surface or chemical modification of the latter and in a dry processing route [60]. Such a route to fabricate CNT-nanoparticles heterostructures will prevent their contamination with unwanted chemicals as well as eliminate cleaning and purification steps. Thus, we fabricated CNT–Au nanoparticles heterostructures by way of dewetting Au films coated onto

Plasma oxidation

CVD process AuOx+e-+C (decomposed Xylene)

83

AuOx+H2

AuOx+e-+C (decomposed Xylene)

Fig. 4 – Schematic illustration of the growth mechanism of GNPs, which shows the following competing phenomena: Ostwald’s ripening effect, surface gold oxide, surface migration of Au species, and CVD reaction kinetics.

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CNTs. CNTs were grown in a xylene–ferrocene CVD growth method [23–25] and then dispersed on a Si wafer for further coating with Au films in a sputtering process for 30 s (Fig. 5A). The selected Au sputtering duration resulted in smallest size of dewetted Au nanoparticles on the Si wafer (30 nm, Table S2, see Supplementary information). In order to prepare CNT–Au nanoparticle heterostructures, dewetting temperature of 400 C was selected because CNTs are known to be stable at this temperature [25]. After the dewetting process, CNTs were uniformly decorated with Au nanoparticles (Fig. 5B). CNTs were minimally damaged and base Si wafer was also uniformly patterned with Au nanoparticles (Fig. S19, see supplementary information). The latter could be due to two reasons: (a) Surface migration of Au from curved CNT surface to flat Si wafer and (b) dewetting of Au films

A

B

100 nm

C

100 nm

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E

100 nm

F ~0.35 nm

20 nm

5 nm

H

G 0.22 nm Au (111) 0.41±0.08 nm

0.41±0.08 nm

0.22 nm Au (111)

Fig. 5 – SEM images of (A) Au film sputtered on CNTs for 30 s before dewetting, (B) CNT–Au nanoparticles heterostructures after dewetting process, and (C, D) CNTs coated with GNPs after CVD growth process. (E–H) TEM images of GNPs decorated on CNTs. Note: Inter-wall spacing for CNTs (F) and CVD-grown graphene shells around Au nanoparticles coated onto CNTs are shown (G and H). The arrows in (B) shows exposed regions of Si wafer and (F) indicates presence of thin amorphous shell on CNTs.

coated onto the exposed regions of Si wafer that were not covered with CNTs. The first one can be attributed to the fact that Au nanoparticles favored low chemical potential regions (low curvature or flat substrate) than high chemical potential CNT surface [25,59]. These chemical potential effects must be dominant, as many CNTs were decorated with aggregated Au nanoparticles on their flat tip regions (Fig. S19E and F, see Supplementary information). The latter was a low chemical potential region as compared to curved CNT sidewalls. Similar has been observed for nanowire decoration with nanoparticles [59]. On the other hand, since the Au coating was performed on the mesh network of dispersed CNTs, Au nanoparticles must have resulted directly from the dewetting of the Au coating on the Si substrate (arrows in Figs. 5B and S19, see Supplementary information). Thus, this approach could be seen as a method to pattern Au nanoparticles on both Si substrate as well as CNTs in a single step dewetting process. However, this is only possible when dispersed CNT mesh on the Si substrate is not densely packed and has exposed Si substrate area beneath. The synthesized CNT–Au nanoparticles heterostructures dispersed on the Si wafer were plasma oxidized and utilized for selective growth of GNPs on the CNTs in a xylene CVD process for 1 h (Figs. 5C–H and S20 and S21, see Supplementary information). CNTs due to the Au nanoparticle coating survived [25] the CVD growth conditions and were also observed to be structurally intact with inter-wall spacing of 0.35 nm after the graphene shell growth. Since surface-oxidized Au nanoparticles selectively resulted in graphene shells, a layer of amorphous carbon on exposed CNT surface is observed (arrows in Fig. 5F). STEM mode imaging and EDS elemental analysis (Fig. S21F and G, see Supplementary information) confirmed the presence of decorated Au nanoparticles and Fe nanoparticles present in the CNT cores. The latter are the catalyst nanoparticles due to xylene-ferrocene CVD growth [23]. Average CNT diameters, Au nanoparticle size, and interparticle spacing on CNTs at different stages of the processing were estimated (Fig. 6). A decreasing trend in the CNT average diameters is observed (Fig. 6A) and in the following order: As synthesized CNTs (43.8 ± 18.2 nm) > CNTs with dewetted Au nanoparticles (29.4 ± 11.9 nm) > CNTs with GNPs (19.9 ± 4.2 nm). This indicates that CNT walls were etched during the dewetting process as well as CVD growth of graphene shells. Such etching of CNT walls has been observed after annealing (at 400 C) in an inert environment [25]. Further reduction in CNT diameters after the CVD growth of GNPs can be attributed to both their vulnerability at high process temperature (>600 C) as well as reducing H2 environment [25,61]. An insignificant change in Au nanoparticle diameters was observed between before (20.8 ± 7.8 nm) and after (24.2 ± 13.0 nm) CVD growth (Fig. 6B). This confirmed that dewetting process resulted in stable sizes of Au nanoparticles, which remain unchanged even after the high temperature GNP growth. Furthermore, a minor change in inter-particle spacing was observed before (8.9 ± 5.1 nm) and after (11.5 ± 5.1 nm) CVD growth (Fig. 6C). Thus, surface migration of Au nanoparticles on CNT surface was highly restricted during the graphene shell CVD growth process. A slight increase in broadness in diameter and inter-particle spacing

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CNT CNT-Au after dewetting CNT-Au-graphene

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5 10 15 20 25 30 35 40 Inter-particle spacing of Au nanoparticles (nm)

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Intensity (a.u.)

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500 0 1000 1200 1400 1600 1800 -1 Raman shift (cm )

2.8 2.4 2.0 1.6 1.2 0.8 1.3±0.1 0.4 0.0 CNT

2.3±0.2 1.6±0.3

CNT-Au CNT-Auannealed graphene Sample

Fig. 6 – Histograms showing (A) diameter distributions of as-synthesized CNTs (black, average 43.8 ± 18.2 nm), annealed CNTs with dewetted Au nanoparticles (red, average 29.4 ± 11.9 nm), and CNTs with GNPs (blue, average 19.9 ± 4.2 nm). (B) Size and (C) inter-particle spacing distributions of dewetted Au nanoparticles on CNTs before (average size 20.8 ± 7.8 nm, average spacing 8.9 ± 5.1 nm) and after CVD growth of graphene shells (average size 24.2 ± 13.0 nm, average spacing 11.5 ± 5.1 nm). (D) Raman spectra for as-synthesized CNTs (black), CNTs with Au nanoparticles after dewetting (red) and CNTs with GNPs after CVD growth (blue). (E) ID/IG ratio estimated using spectra in (D). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

distributions for Au nanoparticles before and after GNP growth could be attributed to the Ostwald’s ripening effect. Raman spectroscopy showed (Fig. 6D) G and D bands centered around 1583 and 1311 cm 1 for samples corresponding to as-grown CNTs, CNTs with dewetted Au nanoparticles, and CNTs with GNPs. Enhancement in the G and D band peak intensities (Fig. S22B and C, see Supplementary information) for CNTs is observed after dewetting and CVD growth process and is mainly due to the presence of plasmonically-active Au nanoparticles [62]. As-grown CNTs showed the lowest ID/IG ratio (Fig. 6E) as compared to after dewetting process, which confirmed that CNT structural damage occurred during the latter process. However, this ratio decreased again after the growth of graphene shells due to their well-ordered and defect-free structure that dominated the Raman spectrum. Thus, the increase in G- and D-band intensities after GNP growth was due to combined effect of plasmonically-active

Au nanoparticles and crystalline graphene shells. In addition, the blue shift (Fig. S22C, see Supplementary information) in the G-band peak location (1590 cm 1) for CNTs coated with GNPs as compared to as-grown CNTs (1581 cm 1) re-affirms our argument about the presence of the strained lattice in the former.

4.

Conclusions

Morphological evolution of Au nanoparticles during the dewetting process was studied as a function of Au film thickness (8–45 nm), annealing time (10 min–10 h), and temperature (150–1100 C). Driven by the minimization of surface and strain energy, most optimized control over size (30– 662 nm) and inter-particle spacing (25–1952 nm) of the dewetted Au nanoparticles was observed during controlled Au film thickness on the Si wafer at 700 C for 2 h. Under these

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conditions, the dewetting process resulted in Au nanoparticle formation within 10 min with thicker films resulting in largersized nanoparticles located farther apart. These dewetted Au nanoparticles were surface plasma oxidized and utilized for the CVD growth of GNPs. With increasing core nanoparticle sizes, thicker graphene or carbon shells were formed, amorphous carbon increased, and significantly smaller-sized (<4 nm) Au nanoparticles were embedded within these shells. The graphene or carbon shell formation around Au nanoparticles restricted their surface mobility on the substrate during the CVD growth process. It was estimated that during the CVD process, the surface migration diffusivity was low (order of 10 17 cm2/sec) and corresponding activation energy (370 kJ/mol) was high for small-sized nanoparticles (30 nm). However, an increasing trend for surface migration diffusivity and decreasing trend for activation energy was estimated with increasing Au nanoparticles sizes, which was consistent with experimental observation of Au nanoparticles size and inter-particle spacings. It is further proposed that this graphene or carbon shells growth mechanism is dependent on surface gold oxide content, Ostwald’s ripening, surface migration of Au species, and competing CVD reactions, which were all influenced by the core Au nanoparticle sizes. As a next step, CNT–Au nanoparticles heterostructures were fabricated by dewetting Au-coated CNTs at 400 C in a dry and surfactant-free processing route. These heterostructures were further utilized for the selective growth of graphene shells around Au nanoparticles coated onto CNTs. CNTs remained structurally intact even after multiple processing steps and a significantly low surface migration of Au nanoparticles on CNTs was observed in both, dewetting and CVD growth process. Overall, the dewetting process eliminated/ minimized aggregation of the Au nanoparticles during CVD growth of graphene shells leading to uniformly patterned GNPs promising for large-area nanodevice applications.

Acknowledgments This work was supported by National Science Foundation (Award #: 0925445). The authors thank the University of Alabama’s Office of sponsored programs, Materials Science and Engineering Program, Research Grant Committee Award for additional support for this research. N.C. also acknowledges Tuskegee University (TU) and Dr. S. Jeelani (V.P. of Research, TU) for supporting L. S. during summer of 2010 through NSF REU funds. The authors thank the Central Analytical Facility (CAF) for electron microscopy equipment, the CAF staff (Mr. R. Martens, Mr. R. Holler, and Mr. J. Goodwin), and MINT Center. The authors are grateful to Dr. R. Reddy, Dr. J. Song, Dr. M. Bakker, and Dr. U. Vaidya for fruitful suggestions as a part of J. Wu’s dissertation committee. The authors thank Dr. S. Kapoor for proof reading the manuscript.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.carbon. 2013.05.055.

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